The present invention relates generally to particulate air filters and is more particularly concerned with a new and improved ultra-high efficiency porous metal filter of the type described.
In the semiconductor and microelectronics manufacturing industry, as well as in other industries, it has become necessary to provide a high purity environment that goes far beyond the conventional capabilities of high efficiency particulate air (HEPA) filters. To achieve this it has been necessary to incorporate filtration systems into the process gases lines employed in these industries. These in-line filtration systems target the elimination of not only particulates, but also moisture and organic materials from the environment where semiconductors and integrated circuits are manufactured and/or assembled. To meet these requirements the filters used in these systems must not only remove all particles present within an incoming or process gas stream but also most avoid inadvertent contamination of the atmosphere or contributing thereto by discharging contaminates as a result of desorption (outgasing) of moisture, oxygen or organic materials.
While thin organic membrane filters have been used successfully for gas filtration in these industries for many years, it has been recognized that an ultra-high efficiency filter of enhanced ruggedness and stability, as well as the ability to withstand elevated temperatures, would be desirable. Enhanced mechanical and chemical stability characteristics have been achieved in some inorganic membrane filtration systems used for filtering liquid streams but thus far such filters have not been satisfactorily produced for use as particulate air filters. The inorganic membrane filters are formed by coating a porous substrate or support with an inorganic membrane coating material. However, such inorganic membrane structures provide a sufficiently small pore size only on the upstream or feed side of the filter and possess much larger pores over the remainder of the filter's thickness. Even thicker more homogenous inorganic filters, such as ceramic or stainless steel filters have not been successful in matching the high efficiency exhibited by the organic membrane filters, namely, filtration efficiency levels greater than 99.999% (referred to as a 5 log reduction in particle penetration since the number of particles penetrating the filter is less than one in 10.sup.5 ).
The ultra-high efficiency particulative filters made from organic membranes have achieved a resistance to particle penetration in excess of a 6 log reduction and even up to and beyond the 9 log reduction level (efficiency greater than 99.9999999%). Organic membranes exhibiting the highest efficiencies have generally been prepared from mixed esters of cellulose, polyvinylidine fluoride or fluorinated hydrocarbons, such as Teflon. However, such materials can not withstand elevated temperatures and are subject to outgassing under certain conditions.
Since even minute traces of gaseous or aerosol impurities may cause haze and defects on a silicon wafer, thus degrading yield and reliability, it is essential that both the process gasses and the environment be maintained in as pure a state as possible by eliminating impurity levels down to less than one part per million (a 6 log reduction) and preferably to less than one part per billion (a 9 log reduction). To succeed in keeping the process gases that clean at their point of use requires not only the use of very clean gas supplies as starting materials but also ultra-clean distribution systems and in-line repurification and filtration at their point of use. Since organic filters exhibit the inherit property of shedding coupled with contaminant outgasing, it is desirable to provide alternative filters that are not subject to these drawbacks yet provide the same ultra-high efficiency levels.
For the more critical processes involving the manufacture of submicron devices, inorganic filters can have many advantages over organic membrane filters. Problems that traditionally plagued organic membrane filters, namely, particle shedding, organic desorption (outgasing), clogging, and thermal degradation can be avoided with inorganic filters. Such materials are structurally strong and do not stretch so pore size can be defined more accurately than with polymeric membranes. Typically, the inorganic filter will not flex in high or pulse flows and therefor is less susceptible to shedding or particle loss. These materials also exhibit long term stability relative to shock, vibration and thermal stress. Such materials can operate under service conditions up to about 450.degree. C., as well as under high pressure conditions. While ceramic filters fulfill many of these desirable characteristics, unfortunately they typically cannot withstand temperatures above 200.degree. due to the presence of teflon seals used thereon. Stainless steel filters made from stainless steel fibers also show potential but likewise fail when measured against the efficiencies of the organic membranes.
It has now been found, in accordance with the present invention, that porous metal filters made as fully homogeneous structures from stainless steel, nickel and nickel based alloys and the like exhibit an efficiency substantially in excess of a 6 log reduction. These filters can be produced for use as process gas in-line filters in state of the art gas supply systems. The porous metal filters of the present invention exhibits long term stability relative to mechanical or thermal stress and operate within the desired ultra-high efficiency levels even under high pressure conditions. As is typical of metal filters they do not exhibit an outgassing problem and there is no particle shedding. Of particular importance is the fact that they exhibit these features within a unit no larger than those conventionally employed heretofore with organic membranes and under substantially identical operating conditions.
Other advantages will be in part obvious and in part pointed out more in detail hereinafter.
A better understanding of the objects, advantages and relationships of the invention will be obtained from the following detailed description of not only the several steps of the process together with the relation of one or more of such steps with respect to each of the others, but also the article possessing the features, properties and relation of elements exemplified therein and in the accompanying drawings which set forth an illustrative embodiment and are indicative of the various ways in which the principles of the invention are employed.